0228. Summary Ranges

https://leetcode.com/problems/summary-ranges

Description

You are given a sorted unique integer array nums.

Return the smallest sorted list of ranges that cover all the numbers in the array exactly. That is, each element of nums is covered by exactly one of the ranges, and there is no integer x such that x is in one of the ranges but not in nums.

Each range [a,b] in the list should be output as:

• "a->b" if a != b

• "a" if a == b

Example 1:

**Input:** nums = [0,1,2,4,5,7]
**Output:** ["0->2","4->5","7"]
**Explanation:** The ranges are:
[0,2] --> "0->2"
[4,5] --> "4->5"
[7,7] --> "7"

Example 2:

**Input:** nums = [0,2,3,4,6,8,9]
**Output:** ["0","2->4","6","8->9"]
**Explanation:** The ranges are:
[0,0] --> "0"
[2,4] --> "2->4"
[6,6] --> "6"
[8,9] --> "8->9"

Example 3:

**Input:** nums = []
**Output:** []

Example 4:

**Input:** nums = [-1]
**Output:** ["-1"]

Example 5:

**Input:** nums = [0]
**Output:** ["0"]

Constraints:

• 0 <= nums.length <= 20

• -231 <= nums[i] <= 231 - 1

• All the values of nums are unique.

• nums is sorted in ascending order.

ac

// in a loop, be careful about the boundary
class Solution {
    public List<String> summaryRanges(int[] nums) {
        List<String> res = new ArrayList<String>();
        //  corner cases
        if (nums.length == 0) return res;

        // two pointers, walk, +1 continue
        int i1 = 1, i2 = 0;
        while (i1 < nums.length) {
            if (nums[i1] == nums[i1-1]+1) {
                i1++;
                continue;
            } else {
                if (i1-i2 > 1) res.add(nums[i2]+"->"+nums[i1-1]);
                else res.add(nums[i2]+"");
                i2 = i1;
                i1++;
            }
        }
        if (i1-i2 > 1) res.add(nums[i2]+"->"+nums[i1-1]);
        else res.add(nums[i2]+"");

        return res;
    }
}

ac2: cleaner for loop

class Solution {
    public List<String> summaryRanges(int[] nums) {
        List<String> res = new ArrayList<String>();
        //  corner cases
        if (nums.length == 0) return res;

        for (int i1 = 0, i2 = 0; i1 < nums.length; i1++) {
            if (i1+1 < nums.length && nums[i1] == nums[i1+1] - 1) continue;
            if (i1 == i2) res.add(nums[i2]+"");
            else res.add(nums[i2]+"->"+nums[i1]);
            i2 = i1+1;
        }

        return res;
    }
}